In thermal processes that occur in industrial furnaces, such as incinerators, cement plants, boilers or others, shell or tubular walls are protected by refractory linings retained by an anchoring system. These anchoring systems are normally made of stainless steel based on iron, chromium and/or nickel, and individual metallic anchors are often the reason of refractory lining failure. In many cases, lifetime of the refractory linings is limited because metallic anchors are damaged, while the refractory material itself is still in good working condition.
Corrosion processes in the furnaces are very complex because many different corrosion mechanisms may occur simultaneously. Furthermore, the conditions often lead to mixed corrosion phenomena. High temperature corrosion (above 300° C.) is the main cause of metallic anchors damages. This corrosion includes two types of corrosion, namely “dry” corrosion and “hot” corrosion. Dry corrosion results from the corrosion of the metal by hot gases, such as for example O2, CO2, H2, chlorine gases (HCl, Cl2) or sulfuric gases (SO2, SO3). Hot corrosion involves the formation of condensed molten salts such as for example sulfates (Na2SO4, K2SO4), chlorides (KCl, NaCl) or carbonates (Na2CO3, K2CO3). The two mentioned types of corrosion may be linked, since molten salts may be produced by reaction of hot gases with atmospheric impurities (Na, K, S, Cl). Other types of corrosion that occur are pitting corrosion, galvanic corrosion, intergranular corrosion and stress corrosion.
It is known to coat anchors with a corrosion-resistant metallic coating, such as alloy coatings comprising Co, Cr, Ni, Al, with additions of Y, W, Nb or Mo, for example. Chromium carbide coating is also known. However, all these coatings may be expensive and difficult to apply.
Known ceramic coatings present the problem that adhesion of the coating on metal surfaces at high temperature is not satisfactory.
The main cause of refractory degradation is the corrosion of metal anchors. Other parts that may be attacked are kiln shells, heater steel constructions, metallic coolers, tubular walls, and steel parts in general.
Another reason for failure of refractory linings is incomplete or uneven/irregular sintering of a refractory lining, for example when the lining is formed using a dry vibratable mix (DVM). DVMs are installed in their final position of use and compacted manually, mechanically, or most commonly by vibration. Refractory linings resulting from such compaction of a DVM are then heated to their final service temperature, which results in a sintering transformation within the composition. During initial batch runs after installation, the sintering of the DVM may not be complete, or be at different stages at different portions of the refractory lining.
The state of the art therefore constitutes a problem.